It should be noted that MPLS networks are described in specifications of the IETF (Internet Engineering Task Force) organisation and are based essentially on packet switching technologies comprising a fixed-length label in correspondence with an IP address. In the long-distance network, these packets follow a path which is generally referred to as an LSP (Label Switched Path). Long-distance networks of the MPLS type which are concerned here are either in non-connected mode, for example based on the so-called LDP (Label Distribution Protocol) or in connected mode, for example of the IP/MPLS-TE type.
FIG. 1 depicts the architecture of a prior art long-distance network, for example of the IP/MPLS type, in this case allowing communication between a user A and a remote user B by means of packets. It includes a plurality of path terminology elements, e.g., routers 1, for example of the IP/MPLS type, interconnected by one or more long-distance transport networks 2.
A long-distance transport network 2 in general comprises one or more point-to-point long-distance transmission systems 3, for example of the optical type with wavelength division multiplexing (WDM) interconnecting different equipments 4, for example add/drop multiplexers 40 (or cross-connect systems) with or without a control plane, but also regeneration equipments (such equipments are not depicted in FIG. 1), which, at the physical level, support a synchronous digital hierarchy, for example of the SONET or SDH type (subsequently referred to as SONET/SDH).
Reference can be made in particular to ITU-T recommendations G.707 and G.783 for further information regarding the SDH hierarchy. Optical networks, supporting a synchronous digital hierarchy, for example SDH, of the type with a control plane, are the subject of the first amendment to ITU-T recommendation G.8080/Y.1304 (2001) entitled: Architecture for the Automatically Switched Optical Network (ASON).
In the terminology of the SDH standard, the communication medium between two termination equipments or path terminating elements, in this case two routers 1, is called a “path, the communication medium between two multiplex equipments 40 or between a termination equipment, in this case a router 1, and a multiplex equipment is called a “multiplex section”, and the communication medium between two regeneration equipments or between a multiplex equipment 40 and a regeneration equipment is called a “regeneration section”. For generality purposes, in the present description, the communication medium between two SONET/SDH equipments will be called a “section” or “connection section”.
Subsequently, it will be considered that the path terminating elements are routers such as the routers 1.
The SONET/SDH equipments 4 are also linked to the IP/MPLS routers 1 and, to do this, the IP/MPLS routers and the SONET/SDH equipments have SONET/SDH interface cards 5 themselves comprising one or more SONET/SDH ports 6. Each SONET/SDH port has an optical transmitter 7 (in general a laser converting a modulated electrical signal into a modulated optical signal) and an optical receiver 8 (in general a photodiode converting a modulated optical signal into a modulated electrical signal). Optical fibres or optical fibre butt joints 9 are generally used to link the optical transmitter 7 of a first SONET/SDH port 6 to the optical receiver 8 of a second SONET/SDH port 6 and to link the optical transmitter 7 of the second SONET/SDH port 6 to the optical receiver 8 of the first SONET/SDH port 6.
WDM type long-distance transmission systems 3 generally comprise, on the one hand, transmitting transponders 10 converting the modulated optical signal on any carrier received from an optical transmitter 7 of a SONET/SDH port 6 into a modulated optical signal whereof the carrier is accurately known and, on the other hand, receiving transponders 11 converting a modulated optical signal whereof the carrier is accurately known received from a transponder 10 into a modulated optical signal on any carrier which is then supplied to an optical receiver 8 of a SONET/SDH port. They also comprise multiplexers/demultiplexers 12 making it possible to transmit over the same optical fibre 13 several modulated optical signals whereof the carriers are different and to separate according to their carrier the modulated optical signals received on the same fibre. They also comprise line amplifiers 14 making it possible to regenerate the optical signal at the end of an optical fibre before reinjecting it into the next optical fibre. Optical fibres or optical fibre butt joints (not referenced) are generally used to link the transmitting transponders 10 to the multiplexers/demultiplexers 12, the multiplexers/demultiplexers 12 to the line amplifiers 14, the line amplifiers 14 to the multiplexers/demultiplexers 12 and the multiplexers/demultiplexers 12 to the receiving transponders 11. Similarly, optical fibres or optical fibre butt joints (not referenced) are generally used to link an optical transmitter 7 of a SONET/SDH port 6 to a transmitting transponder 10 of a long-distance transmission system 3 and to link the optical receiver 8 of the same SONET/SDH port 6 to a receiving transponder 11 of the same long-distance transmission system 3.
Finally, in order to link the user equipments A and B to the IP/MPLS routers 1, the latter, as well as the user equipments A and B, generally have cards 15 that can for example support, at physical layer level, either the Ethernet protocol or the SONET/SDH protocol.
There will now be described in connection with FIG. 2 an IP/MPLS router such as the routers 1 of the IP/MPLS network depicted in FIG. 1. Such an IP/MPLS router generally consists of one or more ports 11 (a single SONET/SDH port is depicted in full in FIG. 2) and a routing unit 20.
A SONET/SDH port 11 comprises an optical receiver 111 generally implemented by a photodiode which converts a modulated optical signal into a digital signal and an optical transmitter 112 generally implemented by an optical laser which converts a digital signal into a modulated optical signal.
It should be noted here that the SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) hierarchies concern systems for synchronous transmission by Synchronous Transport Module (STM for SDH) in the form of synchronous frames, comprising a Multiplex Section Overhead (SOH) and a payload consisting essentially of one or more Virtual Containers (VC) of different hierarchical levels: VC4 and VC3 for high order containers and VC2 and VC1 for low order containers. Each of these virtual containers also comprises a Path Overhead (POH) and a payload generally denoted AU-n (Administrative Unit, level n). It should be noted that reference can be made in particular to ITU-T recommendations G.707 and G.783 for further information regarding the SDH hierarchy.
In a synchronous hierarchy port of an IP/MPLS router such as that depicted in FIG. 2, from the synchronous transport module STM present at the output of the optical receiver 111, its SOH overhead is extracted in an extraction unit 113. Its payload is demultiplexed in a demultiplexing unit 115 in order to recover the virtual containers VC from which the POH overheads are extracted in an extraction unit 117 at the output of which the corresponding administrative units AU-n are available. On the sending side, the administrative units AU-n become virtual containers after addition of a POH overhead in a unit 118, which are then multiplexed in a multiplexing unit 116, it being understood that the virtual containers can be associated by a contiguous or virtual concatenation so as to be grouped together or be used directly, and then become synchronous transport modules after addition of the corresponding SOH overhead. Finally, the transport modules STM are supplied to the transmitter 112.
The routing unit 20 comprises a routing control unit 21, a transfer unit 22 including in particular a routing table 23 based on the IP address (FIB: Forward Information Base) and a routing table 24 based on the MPLS label (LFIB: Label Forward Information Base) and, finally, a unit 25 for assigning logical ports in relation to the physical ports 125 and 126 of the routing unit 20. These physical ports 125 and 126 are linked to the corresponding outputs of the physical ports (not referenced) of the port or ports 11. The transfer of an IP/MPLS packet therefore consists of routing it from a logical input port to a logical output port according to its destination IP address or according to the MPLS label of the LSP path which it marks in the SONET/SDH transport network. The IP routing tables and the MPLS routing tables are generally used during this operation.
The IP routing table 23 in general stores IP prefixes and, for each IP prefix, the identifier of the logical port to which the IP packets whereof the destination IP addresses belong to this IP prefix have to be sent. It can also store IP prefixes and, for each IP prefix, the identifier of the logical port to which the IP packets whereof the destination IP addresses belong to this IP prefix have to be sent, and the MPLS label to be added to the IP packet when the latter enters an MPLS label switched path (LSP) which leads to the destination.
The MPLS routing table 24 in general stores the MPLS labels of LSP paths of the network and, for each MPLS label, the identifier of the logical output port to which an MPLS packet marked with this label has to be sent, and the action to be performed on the label: replacing it by a new MPLS label when the packet under consideration is in LSP path transit and, in this case, including the value of the label of the new LSP path in the MPLS routing table, or deleting this label in the case where the packet leaves the MPLS network.
In general, a logical port is associated with one or more physical ports. Where the physical ports are SONET/SDH ports, a logical port can be associated with one or more unidirectional or bidirectional SONET/SDH paths (path means any concatenated or virtually concatenated virtual container) In the case of unidirectional SONET/SDH paths, the input of a logical port is associated with the output of one or more unidirectional SONET/SDH paths and the output of a logical port is associated with the input of one or more unidirectional SONET/SDH paths. In the case of bidirectional SONET/SDH paths, the input of a logical port is associated with the output of one or more bidirectional SONET/SDH paths and the output of a logical port is associated with the input of the same bidirectional SONET/SDH path or paths.
Where the physical ports are SONET/SDH ports, the IP/MPLS packets are in general encapsulated at the physical level in PPP/HDLC (Point-to-Point Protocol/High-level Data Link Control: see request in RFC1662 comments) frames, themselves encapsulated in SONET/SDH virtual containers (concatenated or virtually concatenated); the IP/MPLS packets can also be encapsulated in other standardised formats, for example using in particular a GFP (Generic Framing Procedure) framing, themselves encapsulated in SONET/SDH virtual containers (concatenated or virtually concatenated).
Each element (equipment, connection section line, etc.) of a long-distance IP/MPLS network or of the long-distance transport network it includes is liable to fail. The most common failures are the result of cutting by civil engineering machines of the optical fibres buried over long distances and which link the different elements of long-distance transmission systems. Other types of failure are possible, in particular hardware malfunction of a network equipment or software malfunction of an IP/MPLS router card.
IP/MPLS routers in general have several mechanisms making it possible to re-establish the IP/MPLS traffic impacted by a malfunction occurring in the long-distance IP/MPLS network. Among others there can be cited IP or MPLS rerouting in non-connected mode, for example based on the LDP protocol, which makes it possible to re-establish IP traffic in general in a few seconds after the failure, MPLS-TE (MPLS Traffic Engineering) restoration which makes it possible to re-establish MPLS traffic in general in a few seconds after the failure, end-to-end MPLS-TE protection which makes it possible in general to re-establish MPLS traffic in a few hundred milliseconds after the failure and MPLS-TE local protection which makes it possible in general to re-establish MPLS traffic in a few tens of milliseconds after the failure.
Generally carried out by the routing units 20 of the IP/MPLS routers 1, IP rerouting consists of recalculating new IP routing tables by deleting from the topology the links impacted by the failure and determining a new path to each destination IP prefix. When the path to a prefix has changed and, therefore, the logical port has become different, the information contained in the IP routing table or tables 23 for this prefix is modified by considering that the IP packets must no longer be sent on the logical ports corresponding to the path impacted by the failure, but to one of the other logical ports corresponding to the new path. The process of updating the IP routing tables in general lasts several seconds since it requires the exchange of IP control packets between the IP/MPLS routers. The rerouting time then corresponds to the time for detecting the fault, the time for propagating the information through the network, the possible delay before recalculation of the new routes, recalculation of the routes, and then to the time for updating the information in the routing tables.
Also generally carried out by the routing units 20 of the IP/MPLS routers 1, MPLS-TE restoration in general consists of recalculating the route of the LSP paths impacted by the failure and replacing the MPLS routing table or tables 24 by one or more new tables comprising, for each MPLS label associated with the identifier of a logical port impacted by the failure, a new logical port identifier and a new MPLS label. MPLS-TE restoration is a little faster than IP rerouting, but can last several seconds. This is because it necessitates waiting for the propagation of the fault information to the head routers of the LPS paths which then recalculate their paths.
Also generally carried out by the routing units 20 of the IP/MPLS routers 1, end-to-end MPLS-TE protection in general consists of replacing the MPLS routing tables 24 by new tables comprising, for each MPLS label associated with the identifier of a logical port impacted by the failure, a backup logical port identifier and a backup MPLS label which are predetermined. MPLS-TE end-to-end protection is faster than MPLS-TE restoration since it does not require recalculation of the route of the LSP paths and since it consists of switching an LSP path to another pre-established protection one, but even so can last several hundred milliseconds (since it is always necessary to wait for propagation of the fault information to the head routers of the MPLS-TE LPS).
Finally, generally carried out this time by the packet switching cards, MPLS-TE local protection in general consists of replacing in the MPLS label tables the identifier of a logical port impacted by the failure by the identifier of a backup logical port and an MPLS label corresponding to the backup section which are also both predetermined. MPLS-TE local protection is generally fast and lasts only a few tens of milliseconds, since it is performed by the router directly upstream of the fault and consists of switching the LSP paths closest to the fault to predetermined backup sections.
As regards SONET/SDH equipments, they also have several mechanisms making it possible to re-establish a SONET/SDH path impacted by a fault occurring in the long-distance transport network. There can be cited in particular, on the one hand, protection mechanisms, such as multiplex section protection, path protection, multiplex section shared ring protection and dynamic restoration, which make it possible to limit to less than 50 milliseconds the interruption of a SONET/SDH path impacted by a failure and, on the other hand, dynamic restoration mechanisms, for example used by a control plane such as that mentioned above with reference to ITU-T recommendation G.8080, which make it possible to limit to a few seconds the interruption of a SONET/SDH path impacted by a failure.
Multiplex section protection consists of duplicating the SONET/SDH synchronous transmission modules and sending them on two disjoint SONET/SDH multiplex sections. If one of the SONET/SDH multiplex sections is faulty, it is the synchronous transmission module received from the other SONET/SDH multiplex section which is used.
Upon detection of a fault in a multiplex section (or a regeneration section) of a SONET/SDH transport network, at least two types of so-called SONET/SDH alarm are sent: SONET/SDH multiplex section alarms (MS) and SONET/SDH path alarms (AU: Administrative Unit). For each of these two types of alarm, direct alarms (referred to as AIS: Alarm Indication Signal) and indirect alarms (referred to as RDI: Remote Defect Indication) are distinguished.
Direct multiplex section alarms (MS-AIS: Multiplex Section-Alarm Indication Signal) are generated by the local unit of a SONET/SDH port of an equipment of the transport network, for example, upon loss of optical signal (LOS: Loss of Signal) on its optical receiver, upon loss of synchronisation of the digital signal, upon loss of the SONET/SDH start of frame (LOF: Loss Of Frame), or for example upon loss of pointer (LOP: Loss Of Pointer). As regards the port 11 of a router 1, the local unit is the unit 119 depicted in FIG. 2.
As regards indirect multiplex section alarms (MS-RDI: Multiplex Section-Remote Defect Indication), these are deduced from information contained in the received multiplex module, for example, in the case of the SDH system, from the value of positions 6, 7 and 8 of byte K2 of the multiplex section overhead (SOH). These indirect multiplex section alarms MS-RDI can be inserted, for example by marking positions 6, 7 and 8 of byte K2, by means of an insertion unit such as the insertion unit 114 of the port 11 depicted in FIG. 2, in a synchronous transport module STM leaving from the port concerned following either reception in the other transmission direction of a direct multiplex section alarm MS-AIS in order to inform, in return, the sending end, or detection by the port itself of an incoming section fault (multiplex section or regeneration section).
The SONET/SDH direct path alarm (AU-AIS: Administrative Unit-Alarm Indication Signal) is generated by any SONET/SDH equipment when no digital signal can be demultiplexed into an administrative unit AU-n (see reference 120). Once such a direct path alarm AU-AIS is detected, it is propagated to the SONET/SDH equipments which are situated downstream in general by setting all the bits of the payload of the container concerned to 1 (an “all 1s” signal is then spoken of), thus avoiding a new detection downstream. So that this propagation can take place as far as the path terminating element, a SONET/SDH direct path alarm AU-AIS is also generated by any equipment upon receiving a path payload whereof all the bits are at 1 (see reference 120′).
As regards the SONET/SDH indirect path alarm (AU-RDI: Administrative Unit-Remote Defect Indication), this is also deduced from information contained in the POH overhead of the corresponding container, for example in the case of the SDH system, from the value of position 5 of byte G1 contained in the POH overhead of each virtual container. These indirect path alarms AU-RDI can be inserted, in an IP/MPLS router, for example by marking position 5 of byte G1, by means of an insertion unit such as the unit 114 depicted in FIG. 2, in the POH overhead of any container of the corresponding path following either reception in the other transmission direction of a direct path alarm AU-AIS in order to inform, in return, the sending end, or detection by the port itself of a demultiplexing fault concerning this path.
The path alarms AU-AIS and AU-RDI are generated by a unit 121.
Thus, the SONET/SDH interface cards of an IP/MPLS router generate two types of SONET/SDH alarm referred to as remote since they are transmitted to the remote SONET/SDH equipment, generally upstream: remote multiplex section alarms (MS-RDI) and SONET/SDH remote path alarms (AU-RDI). These alarms are generally generated in an alarm management unit 122 according to the SONET/SDH alarms generally received on the multiplex section of the same port and generally on the SONET/SDH virtual container in the same position on the multiplex section of the same port.
It should be noted that, for simplification, direct or indirect regeneration section alarms have not been considered here.
The table below indicates (marked with a cross) in which cases in general these remote alarms are generated by the SONET/SDH interface cards of an IP/MPLS router.
Inputs of the management unit 122Outputs of the unit 122MS-AISMS-RDIAU-AISAU-RDIMS-RDIAU-RDIXXXXXXXXXXXX
For example, when an AU-AIS alarm is detected by the unit 121 (upstream of the port), an AU-RDI alarm is inserted in the containers of the corresponding path leaving from the port towards the upstream of said port. Etc.
In the case of an IP/MPLS type communication network, in general, the IP/MPLS routers closest to the element of the network which is the source of a failure on the long-distance IP/MPLS network take the SONET/SDH alarms described above as a basis for activating the IP/MPLS protection/restoration mechanisms also described above for re-establishing the IP/MPLS traffic. As regards the routers remote from this element, they instead in general take the reception of IP/MPLS control messages as a basis.
Thus, for each SONET/SDH path, each SONET/SDH port 11 of an IP/MPLS router 1 must supply the SONET/SDH alarms to the routing units of the same router as soon as they are detected. IP rerouting is in general activated and then carried out by the routing unit or units 20 of the router 1 closest to the faulty element as soon as the SONET/SDH alarm is received by this unit.